Phoenix -- The most stringent restrictions in the nation on the use of abortion drugs were allowed to take effect in Arizona on Tuesday by a federal judge's ruling in the latest in a series of court fights over the state's abortion laws.

U.S. District Judge David C. Bury refused to stop the new rules just hours before they were to take effect. Opponents of the rules said they would continue to challenge the restrictions in court.

Bury made his ruling in response to a lawsuit by Planned Parenthood Arizona and the private abortion clinic Tucson Women's Center, who say the rules severely infringe on a woman's ability to have an abortion. He was asked to grant an injunction that would have blocked the rules from taking effect.

The rules were released in January by the Arizona Department of Health Services. They ban women from taking the most common abortion-inducing drug - RU-486 - after the seventh week of pregnancy. Existing rules allow women to take the abortion pill through nine weeks of pregnancy.

Planned Parenthood estimates that 800 women would have had to get surgical abortions in 2012 if the rules were in effect then. An attorney for the organization also told the judge last week that the new rules could force its Flagstaff abortion clinic to suspend operations. But a spokeswoman said Monday that the organization is still evaluating how it will proceed and that operations at the Flagstaff clinic will continue.

In his ruling, the judge acknowledged that the new rules will make it more difficult for some women in Arizona, especially those in the northern part of the state, to get abortions as they have to travel farther and make more trips to clinics. But he said they aren't obstacles big enough to show that the rules should be blocked.

"The court finds that the injunction is not in the public interest," Bury said.